Nothing, that's why CS became the dominant shooter design. Because it designated one team as the campers, and the other the team that must go in against the campers. I dunno how CS really balances it, because you'd think CT would always have the advantage on cs_maps and Ts would always have the advantage de_maps. I guess the advantage is that the attackers can get an objective win. Also that their are multiple locations, so the attackers can all rush and overwhelm the forces at one objective and then gain the upper hand.
But yeah this was a huge problem every time me and my friends would airsoft at a friends house. The match always took forever because camping is objectively the best strategy. >Hide behind cover somewhere it's hard to see you and wait for someone to come into your line of site and then pop them.
The person that's running around can be seen much easier than he can see the people hiding behind cover. So the match would always be an hour long of everyone hiding and being afraid until someone got bored/bold enough to try assaulting and then campers still win.
I think the only solution is making the game objective base like CS or capture the flag.